1. Field of the Invention
The present invention relates to a frequency-switching oscillator and an electronic device using the same, and more particularly relates to a frequency-switching oscillator for switching oscillation frequencies by switching the feedback capacitance of a resonance system or an amplification system, and to an electronic device using the same.
2. Description of the Related Art
Generally, an oscillator has a resonance system and an amplification system, and the relationship between the resonance system and the amplification system must satisfy conditions for oscillation in order for the oscillator to oscillate. The conditions for oscillation are that the impedance of the amplification system has a negative resistance to compensate the impedance loss of the resonance system. In addition, the imaginary part of the impedance of the resonance system and the imaginary part of the impedance of the amplification system must have reverse signs, and the same absolute values. Further, in the Colpitts oscillator used in the following explanation, it is required for the impedance of the resonance system to be inductive to satisfy a condition of oscillation.
FIG. 8 is a Smith chart showing frequency characteristics of the impedance of the resonance system and the amplification system. The top half portion of the circle shows that the impedance is inductive, and the bottom half portion of the circle shows that the impedance is capacitive. The inner portion of the circle shows that the impedance is a resistance, and the outer portion of the circle shows that the impedance is a negative resistance. Then, the point where the impedance on the right end of the circle is extremely high is called a resonance point. The phase q of the impedance is shown counterclockwise from the right end of the circle from 0 degrees to 360 degrees. In FIG. 8, the solid lines show the frequency characteristics of the impedance. The impedance at a higher frequency is shown as it moves clockwise from an impedance at a certain frequency on the solid line. For example, frequency fb is higher than frequency fa. Furthermore, the impedance at frequency fa and frequency fb is a resistance, and the impedance at frequency fc is a negative resistance. Moreover, the difference in phase of the impedance at frequency fa and at frequency fb is approximately 340 degrees. As an example of phase difference and a resonance point, in the Smith chart, the phase difference between an impedance at a given frequency on the solid line and an impedance at a frequency two cycles to the right from that impedance is 720 degrees, in which case the resonance point is passed twice.
A frequency-switching oscillator is an oscillator which outputs two or more oscillation frequency signals, and conditions for oscillation must be satisfied at each of the different oscillation frequencies. Conventional frequency-switching oscillators comprise a switch element as switching member in the resonance system, and output two or more oscillation frequencies, satisfying the conditions for oscillation at each of the different oscillation frequencies, by switching the state of the switch element.
FIG. 9 shows a conventional frequency-switching oscillator 11. The basic concept of the frequency-switching oscillator 11 shown in FIG. 9 is disclosed in Japanese Unexamined Patent Publication No. 9-307354.
In FIG. 9, the frequency-switching oscillator 11 is a Colpitts oscillator with oscillation frequencies f11 and f12, and is provided with a resonance system 12 and an amplification system 13 being connected.
Firstly, the resonance system 12 has a coil L12, a coil L13, a coil L14, a diode D11, a capacitor C13, and a switching voltage input terminal a15. These elements are important in determining the impedance of the resonance system. One end of the coil L12 is connected via a resonance output terminal a12 to the amplification system 13, and the other end is connected to the anode of the diode D11 and one end of the coil L14. The other end of the coil L14 is connected to the switching voltage input terminal a15, and is grounded via the capacitor C13. The cathode of the diode D11 is grounded via the coil L13.
Then, when a switching voltage is applied to the switching voltage input terminal a15, the diode D11 becomes conductive, thereby operating as a resonator which is termination-grounded by the coil L12 and the coil L13; when no switching voltage is applied, the diode D11 becomes nonconductive, thereby operating as a resonator which is termination-opened by the coil L12. Here, the coil L14 is a choke coil, and C13 is a ground capacitor.
The frequency-switching oscillator 11 is a voltage-controlled oscillator, having a coil L11, a capacitor C11, a capacitor C12, a variable-capacitance diode VD11, and a control voltage input terminal, which are all corresponding to the voltage-controlled portion. The capacitance value of the variable-capacitance diode VD11 is adjusted by a control voltage inputted from the control voltage input terminal via the coil L11 which is a choke coil. The variable-capacitance diode VD11 is connected via the capacitor C12 to one end of the coil L12.
The impedance of the resonance system 12 of such a frequency-switching oscillator 11 is the impedance seen from the resonance output terminal a12 of the resonance system 12 when the frequency-switching oscillator 11 is separated into the resonance system 12 and the amplification system 13.
FIGS. 10A and 10B show frequency characteristics of the impedance of the resonance system 12 using a Smith chart. FIG. 10A shows the impedance when the diode D11 is conductive, and FIG. 10B shows the impedance when the diode D11 is nonconductive. Furthermore, the impedances at oscillation frequencies f11 and f12 are shown by reference numerals f11 and f12.
As shown in FIGS. 10A and 10B, when the diode D11 is conductive or nonconductive as a result of application of a switching voltage to the switching voltage input terminal a15, the impedance of the resonance system 12 greatly changes. FIG. 10A shows the case when a switching voltage is applied to the switching voltage input terminal a15, and the impedance of the resonance system 12 is inductive at f11 and f12. Then, FIG. 10B shows a case when no switching voltage is applied, whereby the impedance of the resonance system 12 is capacitive at f11, and inductive at f12.
Furthermore, in FIG. 9, in the amplification system 13, a transistor TR11 is an amplification element. The collector of the transistor TR11 is connected to a power supply input terminal a14, one end of a capacitor C19, and one end of a capacitor C17, and also is connected via a capacitor C14 to the resonance system 12. The base of the transistor TR11 is connected to the other end of the capacitor C17, and is grounded via the capacitor C15. In addition, a power supply voltage voltage-divided by a resistance R11 and a resistance R12 is input to the base of the transistor TR11. The emitter of transistor TR11 is connected to the other end of the capacitor C19, is grounded via a capacitor C16 and a resistance R13, and is connected via a capacitor C18 to an oscillation output terminal a16. Thus, the amplification system 13 has no switching member, and the frequency characteristics of the impedance of the amplification system 13 are not switched.
The impedance of amplification system 13 is the impedance seen from the oscillation input terminal a13 when the frequency-switching oscillator 11 is separated into the resonance system 12 and the amplification system 13. FIG. 11 shows the impedance of the amplification system 13, and the impedance at the oscillation frequencies f11 and f12 is shown by reference numerals f11 and f12. In FIG. 11, the impedance of the amplification system 13 is a negative resistance at the oscillation frequencies f11 and f12.
Here, the following points can be understood from the impedances of the resonance system 12 and the amplification system 13. Firstly, at the oscillation frequencies f11 and f12 shown in FIG. 10A in the resonance system, the impedance is inductive, and satisfies the conditions for oscillation. Next, the impedances at the oscillation frequencies f11 and f12 of FIG. 11 satisfy the conditions for oscillation by having sufficient negative resistance to compensate the impedance at the oscillation frequencies f11 and f12 shown in FIG. 10A. The impedance at the oscillation frequency f11 of FIG. 11 in the amplification system has sufficient negative resistance to satisfy the conditions for oscillation. However, the impedance at the oscillation frequency f12 of FIG. 11 does not have sufficient negative resistance and therefore does not satisfy the conditions for oscillation. For this reason, when the diode D11 is conductive, i.e. the switch voltage is applied, the conditions for oscillation are only satisfied at the oscillation frequency f11.
The impedance at the oscillation frequency f11 shown in FIG. 10B is capacitive, and does not satisfy the conditions for oscillation. In contrast, the impedance at the oscillation frequency f12 is inductive and satisfies the oscillation conditions. Next, the impedance at the oscillation frequency f12 shown in FIG. 11 has sufficient negative resistance to supplement the impedance at the oscillation frequency f12 shown in FIG. 10B, and satisfies the conditions for oscillation. For this reason, when the diode D11 is nonconductive, the conditions for oscillation are only satisfied at the oscillation frequency f12.
Therefore, when a switching voltage is applied to the switching voltage input terminal a15, the oscillation signal s11 of the frequency-switching oscillator 11 is the oscillation frequency f11. When a switching voltage is not applied to the switching voltage input terminal a15, the oscillation signal s11 switches to the oscillation frequency f12.
According to the conventional frequency-switching oscillator 11, loss resulting from the internal resistance of the diode D11 increases the loss of the resonance system 12, and causes problems such as a drop in the output level, deterioration in the carrier to noise ratio, or the like.
Furthermore, according to the conventional frequency-switching oscillator 11, when it is desired to widen the switch width between the oscillation frequency f11 and the oscillation frequency f12, it can be designed to satisfy the conditions for oscillation by the switching member D11 provided in the resonance system 12. However, since the amplification system 13 does not include a switching member, the range of frequencies in which the impedance of the amplification system 13 has a sufficiently large negative resistance is narrow, making it impossible for the amplification system 13 to satisfy the conditions for oscillation. As a consequence, the conventional frequency-switching oscillator 11, in which the switching member D11 is provided in only the resonance system 12, has a disadvantage that it is difficult to set a large switch width between the oscillation frequencies f11 and f12. Particularly there has been a problem that it is very difficult to satisfy the conditions for oscillation when the frequency switch width has exceeded 500 MHz. When switching member is provided in the amplification system 13, in addition to the switching member D11 provided in the resonance system 12, it is possible to increase the switch width between the oscillation frequencies f11 and f12. However, in so doing, the number of components increases, miniaturization or cost reduction can not be achieved.
Accordingly, it is an object of the present invention to provide a frequency-switching oscillator in which the switching member does not cause loss in the resonance system, there is no drop in the output level, and no deterioration in the carrier to noise ratio.
Furthermore, it is an object of the present invention to provide a frequency-switching oscillator in which the switch width between oscillation frequencies can easily be increased without increasing the number of components, and which can be miniaturized and made inexpensive. In particular, it is an object of the present invention to provide a frequency-switching oscillator which can easily satisfy conditions for oscillation even when the frequency switch width has exceeded 500 MHz, or more.
In order to achieve the above mentioned objects, the frequency-switching oscillator of the present invention comprises a resonance system and an amplification system, for switching between two or more oscillation frequencies, the amplification system comprising a switching member. The switching member switches between two or more oscillation frequencies by switching the impedance of the amplification system so that the impedance of the amplification system satisfies conditions for oscillation at one of the oscillation frequencies, and does not satisfy the conditions for oscillation at other oscillation frequencies.
Preferably, the switching member changes the value of the feedback capacitance of the amplification system, and changes the frequency characteristics of the negative resistance of the impedance of the amplification system.
Preferably, the output of the frequency-switching oscillator is switched between two or more oscillation frequencies by switching only the impedance of the amplification system.
Preferably, the impedance of the resonance system satisfies the conditions for oscillation at two or more oscillation frequencies.
Preferably, the resonance system has a resonant point between the phase of the impedance of the resonance system at one of the oscillation frequencies, and the phase of the impedance of the resonance system at another of the oscillation frequencies.
Preferably, the resonance system has a phase shift circuit for determining the difference between the phase of the impedance of the resonance system at one of the oscillation frequencies, and the phase of the impedance of the resonance system at another of the oscillation frequencies.
Preferably, the resonance system has a resonance circuit, and the phase shift circuit has a coil and two capacitors. One end of the coil is connected to the resonance circuit, and is grounded via one of the capacitors. The other end of the coil is connected to the amplification system, and is grounded via the other capacitor.
Preferably, the resonance system has a resonance output terminal, and the amplification system has an oscillation input terminal, a switch voltage input terminal, and an oscillation output terminal. The oscillation input terminal is connected to the resonance output terminal, a switch voltage is input to the switch voltage input terminal, and an oscillating signal having two or more oscillation frequencies is output from the oscillation output terminal. The switching member is connected to the switch voltage input terminal, and switches the impedance of the amplification system by the switch voltage.
Preferably, the amplification system has an amplifier element and capacitance member, the amplifier element has a first terminal, a second terminal, and a third terminal. The first terminal is grounded in a high frequency band to be used, the second terminal is connected to the resonance system, and the capacitance member and the switching member are connected in series between the third terminal and the second terminal.
Preferably, the capacitance member is a capacitor, the switching member is a diode, one end of the capacitor is connected to the second terminal, the other end of the capacitor is connected to one end of the diode, the other end of the diode is connected to the third terminal, and the switch voltage input terminal is connected to one end of the diode.
Furthermore, an electronic device according to the present invention uses the frequency-switching oscillator described above.
According to the constitution described above, since the frequency-switching oscillator of the present invention comprises a switching member only in the amplification system, there is no loss caused by the switching member in the resonance system. Accordingly the output level does not decrease, and the carrier to noise ratio is good.
Furthermore, since there is a large phase difference between the impedances of the resonance system at each of the oscillation frequency, the carrier to noise ratio of the oscillation signal is good.
Furthermore, according to the frequency-switching oscillator of the present invention, there is a large phase difference between the impedances of the resonance system at each of the oscillation frequency. Consequently, the conditions for oscillation can easily be satisfied because the switching member is provided in the amplification system, even when the switch width between the oscillation frequencies is wide.
Furthermore, since no switching member is provided in the resonance system even when the switch width between the oscillation frequencies is wide, the number of components can be reduced, miniaturization and cost reduction can be achieved.
Furthermore, since the impedance of the resonance system satisfies the conditions for oscillation at two or more oscillation frequencies, there is no need to provide a switching member in the resonance system. As a consequence, the number of components can be reduced, miniaturization and cost reduction can be achieved.
Furthermore, the phase difference between the impedances of the resonance system can be greatly increased by providing a phase shift circuit in the resonance system. As a result, the conditions for oscillation can be easily satisfied, and the carrier to noise ratio can be improved.
Furthermore, since the electronic device of the present invention uses a frequency-switching oscillator in which the output level does not decrease, the carrier to noise ratio is good, and miniaturization and cost reduction are achieved, the electronic device consequently has similar advantages.